1. Field of the Invention
The present invention relates to compositions which contain a non-photocatalytic metal oxide and tocopherol, which are useful, in particular, in the cosmetics, pharmaceutical, veterinary and agri-foodstuffs fields. The present compositions make it possible, in particular, to inhibit the light-induced peroxidation of unsaturated lipids, and more especially lipids of sebaceous origin (sebum).
The compositions of the present invention may take the form of a cream which can be applied either to the face or to the scalp and hair, as well as to the human body. The present compositions can also serve as a base for a lipstick. More specifically, the present invention relates to compositions which contain a zinc oxide and tocopherol acting synergistically.
The present invention also relates to a method of combating and/or preventing skin irritations, skin inflammations, acne and immunosuppression, as well as to a method for combating and/or preventing the signs of aging. Such skin phenomena are, in particular, induced by solar radiation. The present invention further relates to a process for preserving products containing a photooxidizable lipid.
2. Discussion of the Background
The lipids occurring at the surface of the skin, scalp and hair are known to be continuously subjected to damaging external agents, in particular air, atmospheric pollutants and visible and most especially ultraviolet (UV) radiation.
These lipids are those which form part of the constituents of the skin or hair, as well as those which are secreted by the skin including the scalp, and/or those which are deposited on the skin or hair when products containing lipids are applied to the skin or hair.
The lipids most exposed to damaging external agents are those contained in the fatty secretions of the skin such as sebum, which is rich in squalene. The presence of six double bonds in squalene makes squalene sensitive to oxidation. Thus, on prolonged exposure to UV, squalene is photoperoxidized to give squalene peroxides.
This high production of squalene peroxides causes, in particular, a series of sequential degradations especially in and on the skin, giving rise to many skin disorders. Thus, these squalene peroxides participate in:
(i) the pathogenesis of acne, as described by Saint Leger et al (see British Journal of Dermatology, vol. 114, pp. 535-542 (1986) who point out that squalene peroxides are comedogenic;
(ii) premature skin aging, as described by Keiko OH Sawa et al (see The Journal of Toxicology Sciences, vol. 19, pp. 151-159 (1984)) who discuss the consequences of sun-induced skin burns;
(iii) irritation phenomena, as reported by Takayoshi Tanaka et al (see J. Clin. Biochem. Natr., vol. 1, pp. 201-207 (1986)) who draw attention to the damage caused, in particular, by the repeated use of some shampoos;
(iv) the production of malodorous volatile products (aldehydes, ketones, acids, and the like); and
(v) immunosuppression of biochemical messengers of the biological effects of UV irradiation of the skin, as described by M. Picardo et al. (see Photodermatol. Photoimmunol. Photomed., vol. 3, pp. 105-110 (1991)).
In order to limit the peroxidation of unsaturated lipids, it is known to apply to the skin photoprotective compositions containing at least one anti-free-radical agent and at least one screening agent.
This is the case, for example, with the complex composition described in FR-A 2666226 relating to a cream for combating photon intolerance, and which contains alpha-tocopherol among other anti-free-radical constituents and titanium oxide among several physical and chemical screening agents present in the composition. This composition has the drawback of affording little protection against ultraviolet radiation and of being unsuitable for the cosmetics and dermatological fields on account of problems associated with the "photocatalytic" activity of titanium oxide, as explained below.
In effect, the inventors have found, according to the headspace method, that titanium oxide, under ultraviolet exposure or after a few hours at 37.degree. C., catalyses the production of peroxide radicals from the lipid constituents contained in cosmetic compositions. On application of these compositions to the skin, this gives rise to harmful effects such as inflammation. For a description of this method, reference may be made especially to the publications of Q. L. N'guyen et al, Symposium of AFECG-SFC, Bordeaux, May 1984, pp. 358-359, "Evaluation de l'oxydation aldehydique dans les produits cosmetiques" "Evaluation of aldehyde oxidation in cosmetic products"!; and of K. Warner et al, "Pentane formation and rancidity in vegetable oils", Journal of Food Science, vol. 39, pp. 761-765 (1974).
For this reason, in order to overcome the drawbacks associated with the use of titanium oxide, surface treatments of titanium oxide have been suggested, for example in WO-A-90/09777. However, such treatments do not enable the photocatalytic activity of titanium oxide to be decreased sufficiently, and use of the treated titanium can also cause skin damage on exposure to UV.
The beneficial properties of zinc oxide for the skin are, moreover, known. In particular, zinc oxide retains greases and moisture, absorbs UV, possesses good covering power and is, in addition, soothing (see, for example, the publication of L. D. Grady appearing in The Journal of the Society of Cosmetic Chemistry: 1947, July, volume 1). Thus, zinc oxide has been incorporated in cosmetic compositions such as face powders (see the above publication of L. D. Grady).
More recently, the company NIVEA has marketed a composition in mask form called "Masque 3 minutes Hydropurifiant", which contains, among other ingredients, zinc oxide for its astringent properties, tocopherol acetate for its activity in cell regeneration and kaolin to absorb the excess sebum. This composition is not designed to inhibit the photoperoxidation of lipids, and in particular squalene, and is not capable of inhibiting this photoperoxidation.
An anti-sun cream based on zinc oxide and about ten other constituents including an antioxidant is disclosed in GB-A-2,184,356. However, the use of a tocopherol as an antioxidant for the purpose of combating the photoperoxidation of unsaturated lipids, in particular those of sebaceous origin such as squalene is not taught.
Moreover, it is known from EP-A-579,078 to combine zinc oxide and tocopherol, but this document neither teaches nor suggests the inhibition of the photoperoxidation of unsaturated lipids by this combination, particularly since, in the examples, tocopherol is used in its acetate form, a form which is not capable of inhibiting this photoperoxidation.
Thus, the compositions known to date confer insufficient or even zero protection of the skin against the peroxidation of lipids of and on the skin.
Accordingly, there remains a need for compositions capable of inhibiting the active forms of oxygen, and in particular displaying effective activity in combating lipid peroxidation, in particular in combating squalene peroxidation, on exposure to ultraviolet light and/or to other oxidizing factors, and which is well tolerated by the skin and/or hair.
In the context of the present invention, the term "active forms of oxygen" is understood to mean forms of oxygen, free-radical or otherwise, such as, in particular, ROOH (R representing, in particular, the hydrocarbon chain of an unsaturated lipid), OH.sup..circle-solid., ROO.sup..circle-solid., O.sub.2 H.sup..circle-solid., .sup.1 O.sub.2 and more especially ROOH where R is the hydrocarbon chain of squalene.